General question on rfc1918

Hi there, I just had a real quick question. I hope this is found to be on topic. Is it to be expected to see rfc1918 src'd packets coming from transit carriers? We have filters in place on our edge (obviously) but should we be seeing traffic from 192.168.0.0 and 10.0.0.0 et cetera hitting our transit interfaces? I guess I'm not sure why large carrier networks wouldn't simply filter this in their core? Thanks, -Drew