Dent appearing on horses face?

[Luci07]

This is bizarre, not there last night. My horse was what looks like a dent in his left cheek. You can clearly feel the dip. He is currently turned out solo, has no temperature, no cuts or other bruising, lunged him tonight and he is as bright as he always is. Horse is a 6 year ISH. My YO is going to get the vet to check it but we were stumped. Thoughts? YO had a good look inside his mouth and can't see anything?

 

[Britestar]

Has he previously had sinus surgery or teeth removed by bone flap? My boy has a large dent in his face due to tooth removal.

 

[Luci07]

No he hasn't...I have had him for a year.

 

[Moomin1]

Possibly start of an abscess?