If your husband has an issue with you asking him who he might have had lunch with and why he didn't feel the need to mention it then let him STEW.
He may get aggressive... Guess what... STand your ground...
If he gets aggressive :
a. He's a bully b. He's fostering an unhealthy climate for your home c. He's got issues d. He may very well be hiding something
Either way, its on HIS head not yours.
He's been talking about leaving, looking at apartments, won't go to family therapy, keeping his cell phone in his shower... He's been doing everything that would justify you asking for an explanation for the lunch...
He has NO REASON to get defensive right now... HE set this climate UP so HE has to deal with the discomfort and YOUR PANIC.
If my wife feels unsafe because I am threatening to leave my home, she has every right to question me about mysterious charges to my credit card... I set the climate of the home up with my behaviuor... SO I have to suck that up until I change it.
If you want to play it safe and let it slide and keep an eye on him that's fine... Your call...
I just dont' want you to feel like you are crossing a line by bringing it up... You most certainly aren't.
