Yeah, remember Mutt mentioned the warning sign about grave robbers and Indy said "good thing we're not..." Plus at the diner Mutt, mentioning that his mom told him Indy was some kind of grave robber?
Indy saw the dagger and handed Mutt his switchblade and began to pocket the dagger... Mutt cleared his throat and Indy was embarrassed so he made the excuse - "I didn't want to keep borrowing yours all the time... (beat) I was gonna put it back."
Indy felt embarrassed about taking the dagger in front of Mutt who he just told "good thing we're not grave robbers."
I liked this scene a lot.
It's the subtle humor that always works best in Indy movies.