I agree it does sounds very suspicious. I don't want the OP to start feeling unnecessarily insecure about her relationship. It could very well be a case of phone on silent and he didn't check it for a while.
But it would be enough to get me thinking and playing detective just in case.
Either way I think it is time OP trusts her instincts. Maybe he is doing the dirty on her with someone within walking distance or was picked up, or maybe there are other issues at play and stress is getting the better of him and he just had to get out of the house.
Hopefully it is all innocent.