Bet you $5, the magnet, looks similar to a washer, that is glued or epoxied to bottom of the oil level float sensor inside of the oil tank has come off the float. The oil level float in the tank "floats" on top of the oil. It slides up and down a plastic guide with the fluctuating oil levels in the tank. When the low oil alarm sounds, the float magnet is close enough to the bottom of the oil tank for it to trip the alarm switch. Once you put enough oil back in the tank, the float/magnet rise causing the low oil alarm switch to automatically reset. Long story short, if the magnet seperates from the float it will fall to the bottom of the tank and trip the low oil alarm even if the oil tank is completely full. Tried to reattach mine multiple times but was unsuccessful. All parts are located inside the tank and can't be removed... finally just went to local marina service department and ordered OE replacement oil tank. Cost me about $50 for a 1/2 gallon tank and took me around 10min to install.
Below this is the link to a short video I found on youtube. Very informative on the inner workings/components of the oil tank. Definitely don't suggest trying to fix it yourself. Just buy a replacement tank and fix it the right way.
https://youtu.be/mwmm-ZZtBM0