Probably water pump if it is original. Could be a loose hose clamp or hose, heat exchanger/ exchanger gasket- this must be changed every couple of years.

grommets for the heat exchanger and or the water circ pump and or the hose to the water circ pump.

I had a similar experience the right after I bought my boat. Prepping for the first run of the year I noticed that the coolant tank was empty so I filled it up and about an hour later the coolant was on the driveway below the drive. Long story short there were small rocks on one side of the heat exchanger so when it was winterized there was water not anti-freeze in those tubes they cracked allowing the coolant to run out thru the exchanger down thru the water pump and out the pickups. If your plugs are out it would come out those and into your bilge.

but tech speaking if one only uses the air pump to drain their 496 thats is improper complete winterization. Even if the heat exchanger is plugged up it still can drain all the water out when drained properly. Just saying --

the air pump does not drain all the system fully anyways and at times being an open raw water system, those sides become scaled - sand - corrosion up that causes water restrictions and draining issues. Clearing and cleaning out the heat exchanger should be done every year because it part of the yearly maintenance. It is best to do at lay up

heat exchanger grommet(s) been there done that
don't even need to change it if all is well otherwise
they just shrink in the cold and loose seal.
grow back in the spring and seal fine
don't overthink it if there is no other indication of a problem

Same here. Merc had a bulletin out on the issue that involved loosening the hose clamps and slightly repositioning the heat exchanger if memory serves. Mine would always lose a few ounces of coolant every year regardless of how I positioned the heat exchanger or how tight or loose i had the hose clamps.

no matter of temps and your leaking AF whenever is not normal - the grommets need to be changed. When the stuff was new it does not do that period. That is not normal as the rubber / polymers in the grommets have been comped by father time just like a 16 year old tire stored in garage is not good anymore.

Canada and Alaska boaters would never have AF in their engines if this was normal -- leaking AF in the cold from rubber components shrinking and needing to swell back up when warm. That is a sign your gromments and or whatever causing a leak needs to be address and address with new seals / gromments or whatever.

Change the gromments out because on the water thats when they will completely let go and dump AF all over. - heat soak and cooling cycles from hot to cold also degrade rubber by products. They become hard and or dried out. Marine engines suffer the worst because they have no air hitting the engine plus condensation can happen in a tight sealed engine compartment making matters worse.

everything has a life span. 10 years for boat parts is about the overall average on many items

Mine did it from day one. I was the original owner. I know Merc had a TSB out on it.