Air carrier Ramp Lead reported mechanics started an engine with ramp personnel and equipment in close proximity.

Narrative

Myself; Lead; and 2 agents were working a turnaround flight in extremely inclement weather. We were in the process of unloading luggage for Aircraft X with a belt loader lined up to the aft compartment and 2 luggage carts lined up to the belt loader. Myself and 1 agent were scanning and loading bags onto the belt loader and 1 agent in the aft stacking luggage. We were moving bags from one of the carts to the forward compartment when I noticed the beacon suddenly started flashing. I made several comments about it to my Agent. I then noticed the aircraft parked at Gate XX had its strobe lights on; so I assumed this may be due to the lowered visibility conditions. Shortly after the beacon turned on; while still loading bags; I began noticing an uptick in noise.I knew the APU was already running; so I thought what I was hearing was the air condition system kicking on. I quickly realized that it was sounding more like an engine; but figured it must be from the plane on [Gate] XX with the strobe lights. I moved to the other side of the aircraft near Engine #2 and noticed it was fully spooling. I see no wing walkers. Nobody with wands standing at the aft of the aircraft giving the engines running signal. Nothing. I immediately called over the Supervisor and they had nothing to offer other than agreeing with me when I said I would be filing a report on this as soon as I get home. I then went into the flight deck to speak with the 2 mechanics and explain to them that they just had an engine spooling while we were servicing the aircraft and that I will be filing a safety report. The one Mechanic; asked me if you saw the beacon on; wouldn't you assume the engines running? I simply said you can explain your side when the time is necessary and have a good day. To recap; an engine was started within feet of one of my agents working in the bin of the aircraft. Had they started Engine #1; this may have been a much more gruesome outcome. They had zero spotters. Nobody waving wands warning of an engine running. We had a belt loader and 2 carts next to the aircraft; yet another Mechanic who showed up told my Agent they didn't see us working on the flight. I took pictures from all angles of the gate and challenge anyone to not notice the equipment. Then; the completely unbothered response and unbelievable disregard for the seriousness of the situation by the mechanics was frankly concerning. As professionals that have access to flight decks and aircraft turbine engines; any display of reckless behavior without immediate remorse should be terminated from their position. I unfortunately didn't get a picture or video of the engine running as I was busy trying to get a Supervisor to the gate as soon as possible. Also; I had to answer the question about which Manager I reported to. However; I am not 100% sure if that is the actual Supervisor. I do know they were the Supervisor but unsure of their last name.