<human>: Me [30F] with my SO [29M], been together a year. He had a bitter breakup with his ex about a year before we started dating. (She broke up with him, he was devasated). 

She has started texting him again recently. He did not tell me this, I checked his phone yesterday. Can't explain it but had a bad feeling. He's lied about random inconsequential things before, long story. 

Anyway, looks like it started innoccous, work related stuff (we all work at the same place, but in varied departments...he and I don't see each other during the day b/c we're in different buildings, but they work in the same building and see each other somewhat often). She's clearly been reaching out to him trying to be friendly again. She even offered to take him out for a beer, he did not respond. In fact, it looks like his texts were very appropriate up a day ago when he randomly texts her talking about old times, saying it seems like forever ago, etc, remember this and that, etc. She wrote back something along the lines of yeah, wasn't that fun, etc. It ended there and went back to work stuff. 

They haven't texted often (maybe 4 or five interactions over the last 5 or 6 months) but this last one from him kind of threw me. No other problems in our relationship. He seems to be very in love with me and is generally a really good partner.

Can you give me a shortened version of the text?
<bot>: Boyfriend texting ex about old times, but I only know b/c I snooped. Is this a red flag and should i say anything?