Found a women's sock in my boyfriends drawer?

I'm gonna try and fit this in a nutshell. Me and my boyfriend were long distance (2 hour drive), but now I live with him. Before I lived with him, I used to come over for a few weeks at a time so to keep myself busy while he was at work (I work from home) I did washing and cleaning etc.

He has a habit of leaving washing until the last minute so I have often seen his underwear drawer empty, before I moved in there was no sock there.

I did our washing yesterday, was folding it an putting it away and I go to put his boxers away and I find this black women's trainer sock with frills on the top of it.

I asked him about this sock this morning and he just says he doesn't know and then stopped talking, he didn't even seem shocked that it was there or ask "what sock?" And he's normally one of those guys who needs to figure everything out.

It must've got there somehow and I certainly didn't move it?

Also before anyone asks, the only time I ever had socks like this was for my dads wedding when I was about 8 and they were white.

Update: I asked him about it again, he messaged me saying he's at a loss, I fell asleep but after woke up and replied saying "you and me both" but I didn't put any emoticons and he knows I only ever do that when I'm upset however I only did it that time because I was half asleep. I sent that and went in the shower. He replies asking if I'm okay and telling me that he loves me but I'm in the shower so I don't see the message, he then rings me to check on me and I don't answer, he rings 6 more times because at this point he's starting to worry. I get out the shower, see the phone calls and the messages and message him back saying that I'm okay and I love him too, then asked him if every thing is okay due to the 7 phone calls. And about 30 seconds after I send that, I see his car pull up outside and him basically running from the car to the front door, straight upstairs to where I was. He never comes home from work during the day but he said he was so worried about me, he had to check that I was okay. He gave me a hug and we talked about the sock, he said it was probably his ex's (they broke up about 2 years ago) and that some of her stuff is still dotted around in places and said that we can burn it if I like which made me laugh and then he threw it in the bin. Double checked that I'm okay before he left and then went back to work.

Luckily for me I have a nice ending to this! Thank you for your comments, they really made me feel supported when I needed it most :)