Weird marks on t shirts

[H930]

Hi everyone,

I’ve noticed these weird marks/stains on lots of my husband’s t shirts - what could they be? I’m thinking maybe deodorant but they’re either side at the bottom/up to half way up….he doesn’t know what it could be….but it’s ruining his clothes and I can’t seem to get the marks washed out!

Any ideas??

What sort of fabric?
What detergent do you use?

These are cotton t shirts, I wash them with Fairy non bio powder, the whites and colours I use Vanish powder also

Do you wash them before he first wears them from new?