My husband is a computer geek, and usually is on his computer when he's supposed to be "watching the kids". Our youngest is now 2-1/2, and LOVES to climb on things. He realized one afternoon that he had to quit being oblivious to what the girls were doing when the 2 year old had climbed up on to the kitchen island, and I hear, "OH MY GOD!!!" Of course, I got to tell him, "She wouldn't have gotten up there if you had been paying attention to her."
Plus she also loves to play in the toilet. I don't know why...our 5 year old didn't. BUT she'll unspool the TP, throw it all in there, play with the brush, etc etc until the bathroom and she are wet all over. It didn't take long to realize that if you can't see her or hear her, then it's time to figure out where she is and what she's doing, because chances are, she's doing something she isn't supposed to be doing.
Eventually he'll get the idea - but no you aren't over-reacting, and someone suggested bringing it up at the next dr. appt. so he can hear how important it is - that's a good idea. Any time you are around other couples and can ask, "Did the kids ever do anything they weren't supposed to before you guys realized what was going on?" and those should help instill the importance of paying more attention to her.
Good luck!