Exploit, hacking or something else?

I just got done playing a doublecross match (Valve sever) and I was killed a few times by a sniper using the classic. It was to be expected as I was also playing sniper but all the times I died to them the killcam showed them as invisible (except for the classics laser), the first time it happened I checked the kill feed to see if they died right as they killed me (they hadn't) and then I checked to see if they had left the game right as they killed me (they hadn't) I didn't get a screenshot because I was just so confused and had only died to them 2 times before the match ended. Is this some well known exploit on doublecross or is it something else?