Primary purpose of core plugs is to fill up the holes left in the casting so you can extract the sand, or whatever, core used to define the inside parts of the casting. Mostly waterways. Drive in plug is just the easiest way of doing the job. Make it the right shape and it has enough clearance to slip in easily yet a swift blow in the middle will cause it to expand evenly to grip all round for a nice seal. Popping out for pressure relief on freezing or very serious overheating is an incidental and very unreliable benefit.
Hate to say it but your droplets look very similar to the ones I had when spending 4 years, but only 12,000 miles, chasing a small water loss. In my experience unless the leak is pretty big, as in stop at regular intervals to top up size, visible droplets and tracks have more to do with under bonnet airflow and where the fan blows stuff than the actual location of the real leak. I fixed several small leaks that ought to have been about right for the coolant lost to little effect.
Even a good scrub down and a UV kit didn't really solve things although I got very suspicious of the water-pump. But I "knew" that was good cos it got a good inspection when I changed the (bodged) waiter pump gasket during a crankshaft seal change. Eventually it dumped enough overnight to be really sure that it was the pump seal. With 20/20 hindsight it was clear that the pump seal had been failing for a long time. Those things are pressure loaded so it would hold up once the car was up to temperature and the system properly pressurised but would leak during warm-up. So a larger, but still not huge, short term leak looked like a small all the time one.
Clive