Completely understandable that you do not want to place a VAR in her car for the reason you stated. I like ShutterHappy's suggestion on getting a PI involved. If anyone is going to know how to install one correctly and securely with regard to laws in your state, it would be a PI. At least talk to one to get a feel on whether it is a low risk move. If the PI tells you "Oh no, we NEVER use VARs! Too illegal!" Then thats confirmation for you of your own risk level.
But, you said your wife isn't tech savy, correct? Would she even recognize a VAR if she saw one? Maybe you can disguise the VAR to make it look like a small digital ODBII scanner. Get a label maker and print some stickers labeled like "Scan" for record button, "Store" for the stop button, print the brand name also in case she Googles it up she'll see it is a maker if ODB scanners. There's probably a headphone jack on the VAR you can use to plug in a cheap set of head phones but cut short the wires and chuck the earbuds. Make it look like the serial connector was damaged. You'll want to insert the plug anyway since it would mute the VAR of any accidental play back sounds.
Then insert the VAR into the car per PI's instructions. If your wife somehow did find the VAR you can just tell her "Holy crap I thought I lost my ODBII scanner!!! Where was it? Under the seat? It must have fallen out of my pocket and rolled under there when I took your car to the parts store that day! Oh, thank God you found it for me, hon!" These things are expensive!"
Also, when it comes time to pull the VAR and your wife catches you and ask what you are doing in her car, you can also say "oh, I was just looking for my ODBII scanner and thought I might have left it in your car when I went out to get parts for my car..(waving VAR in air)..and I found it!"
Just know that to cheaters, their own cars are perceived to be the safest place to have secret conversations. If your wife doesn't do a whole lot with her phone with chat apps and texting as evidence in your phone bill then conversations are likely being held in her car and in person with the AP or friends in the know of an affair.
Now that you've raised suspicion she's probably just backing off the inappropriate friendship of whomever it is she's toying with on that slippery slope. You want to find something, whether there is or is not anything going on it will require a little patience. Be eyes wide open to observe and take notes. Relax at home and with her so she falls back into habit. Then, see if she feels it is safe to go out earlier or stay late again, all dressed up as she did before.
This make take a couple of weeks to a couple of months. In that time you should consider seeing a divorce attorney NOT to file now but to prepare yourself in the event that is necessary. Now is the time to be fully knowledgable with your options should you find that she is in an affair. There is a ton of info in the healing library to get you prepared, either for D or R. The best place you want to be if indeed you have solid proof of an affair is to be well informed of a likely outcome in a divorce, well informed of typical wayward behavior (cheater's script) up on exposure, and well informed on identifying regret from remorse from her upon exposure. Most of all, you will have time to contemplate what would be a dealbreaker for you. Is a PA or even an EA too far you to deal with? Would you consider R if she moved mountains too help you heal?
I hope that you find out that all along she is just adjusting to you being home more. I hope she is dressing up to show off to you the progress at the gym. I just find it odd that she would do this after you two argue. When my wife and I argue she changes into her pajama pants, grabs something to eat and watches greys anatomy or Outlander to calm down.
Just a thought, maybe you get the book "5 Love Languages" by Gary Chapman and invite her to read it with you. That might give you a some indicator as to where her head and heart is at with the marriage. Cheaters don't like to read relationship improvement books because they think:
The problem is not with them, it is with their partner (as their own rationalizations to cheat have convinced them)
They like the relationship just the way it is - you to be comfort and convenience while my AP for romance and excitement. "I've got my cake. It's not my fault you dont have yours."
That book is like a mirror. Its going to tell them exactly what they dont want confirmed, that what they are doing is wrong...but this affair feels so right...
They are more reasons but you get the drift. If the relationship is not worth it to them it is not worth spending time to invest in, no matter the reasons.
Thus, I think her opting to spend time with you is a good thing. Keep up the invites and bring that book into conversation. You may not get anything on the VAR but you'll get some feedback on her willing engagements with you.