05 Mustang GT. Put battery back in after 3 weeks now having electrical issues.

Dont drive the car often so took the battery out and put it on a tender. After 3 weeks of battery out i put it back in and drove it. Everything was fine till i got back home. Went out to the car a few hours later and my headlights were on. Had to turn the car over to turn them off even thought they were turned off. Few more hours later my passenger side headlight came on and when trying to turn it off it started flickering. Decided to remove the negative cable, while doing so the same light started flickering off and on.

Any idea whats causing this? Everything worked fine before i took the battery out for so long. Thanks for any suggestions!