Husband home late last night, about 12:30 after a work do. Comes to bed at 1 and falls asleep and his phone beeps with a text at 1:15 which wakes me up. It's from a woman I've never heard of, a colleague from work I guess - its short & quite innocuous about a work thing but the late timing makes me suspicious, so I unlock his phone and read a series of slightly flirty texts between them that night which are again work based on the face of it, but hint at a planned evening get together in a couple of weeks after work. In the morning I ask him who texted at 1:15 as it woke me up - he's already on his phone by this time - and he says it was his sister and shows me. And yes the last text was from his sister. So he had already deleted the exchange from the night before.
AIBU to be upset by this? I don't feel proud about checking his phone but the fact that he was so quick to lie about who texted him makes me feel very suspicious and sad.