I loaded up GTA Online and since this morning when I was playing, my character’s developed a yin and yang tattoo on his hand. Why and where has this come from?
Why has the GTA Online character suddenly got a hand tattoo
grand-theft-auto-online
Related Solutions
You are only allowed to keep weapons you have purchased in Ammunation. Weapons you have picked up during missions etc are only available until you log off from GTA Online.
You also lose the weapons if you die or change session.
A similar thing happened to me, but in a worse condition - all my progress was gone; my rank was reset to 1, and i lost my cars, and apartment. I recommend you to contact rockstar support, by posting a question at https://support.rockstargames.com/hc/en-us . They restored my progress within 5 hours I posted the question.... Here's the mail i received- 
Best Answer
If you have another person in your household that plays on your account,they most likely have added it.You can get it removed,but not refunded.Hope this helps.