Your original suspicions were based on coincidental log-in times for Messenger.
Your present suspicions are based on a drop in sexual frequency from weekly to daily back to weekly.
You have posters telling you to trust your gut. I’m a trained investigator – a former cop – and I can tell you 100% that I would NEVER even contemplate facing a judge and tell him that my “gut” told me the guy in the stand was guilty. At best the gut might tell you that you need to investigate further. Something we suggested and offered advice on how to do in your original post. This is what I shared there:
I still think the Messenger offline timing is extremely weak and honestly I don’t see that you have the “proof” some mention.
You have indicators. You have red flags that need to be checked, but anyone telling you that you have more is reading something that isn’t there from your lines. I would not want to base potentially life-altering decision on such a weak base.
The white truck? Definitely something to look into. Just keep in mind white is the most common color with over 30% of all trucks (irrespective of brand or make) being white.
Do you have any chance of monitoring her during lunch? Is there any way you could borrow a colleagues car to sit outside her place of work and try to catch her?
Do you know OM truck type and model?
Do you know OM? Does your wife know you know OM? As in are you all acquaintances or in the same social group? If you were to ask your wife what she was doing in Fred’s truck would she know you know who Fred is?
Are you two in any way stuck home due to covid? This might open some investigative suggestions.
Messenger logs are saved centrally so if she has Messenger on the phone she leaves open to you then have you tried looking at her logs and past conversations?
If they are meeting, then at what times do you think it is? Other than lunch are there times she’s not at work but not at home? Like does she claim to go jogging or the gym?
If she had a burner phone then where would she be likely to hide it?
Does she have her own vehicle? Have you searched it?
Considered placing VAR’s in places where she might be contacting OM?
Keep in mind that if he’s not married then he has the ability to a) talk to her whenever she wants to talk and b) have her over to his place. So if your wife is hanging out at the end of the garden on her phone or spends an hour in the bathroom it could be to talk to him.
Keep a couple of things in mind:
Investigate to discover the truth – not to discover an affair. Maybe the truth is an affair and then that is what you discover, but don’t go searching specifically for an affair.
If after a certain reasonable time you don’t find anything suspicious then maybe there is nothing going on.
-----------
edited to add:
If you have free access to her phone you might search for and install a silent tracker app on her phone. Maybe use the weekend to download and test on your phone: want one that is relatively unobtrusive, quiet and not noticeable.
Look – IF there is an affair going on and IF it’s this WhiteTruck OM and IF he’s single then there are some assumptions we can make:
He can have her at his place for sex (if it has gotten that far). Why take the chance of being picked up by cops for making out in the truck or paying money for a motel if the home is empty 20 minutes away?
He can be contacted whenever she wants to call – this matters in that she will initiate calls and therefore you seek places she feels safe talking in.
With a tracker in place on her phone then if she leaves work for lunch you can head directly to his home-address or follow the tracker.
If you are going to use coincendal log-in times and frequency of sex as indicators of infidelity you might as well also get some chicken-bones and Tarot cards and maybe a crystal ball.
Personally I would rather be more realistic and maybe follow some of the suggestions offered.