Battery Draining Issue

My battery failed on me yesterday (2023 R3GT). It has been off the bike over the winter and charged periodically on an Optimiser tender. It wasnt left on it all the time, just connected for 48 hours every couple of weeks to do a couple of full cycles showing it was fully charged and in good health.

It's been absolutely fine since I out the bike back on the road again at the end of March. No problems or sluggish starting.
Yesterday I rode the bike all day. I had stopped for a break and restarted again twice without issue. Then, after stopping and a filling station to fuel up I went to start the engine. Dash lights were on, fuel sender whirring as normal. Pressed the starter and it clicked, the dash lights went out and it wouldn't turn over. Classic indication of a flat battery. No warning lights showing prior to the failure.
I got a jump start from a kindly local chap (who went home to get his jump leads) and it fired up immediately and rode the 40 miles home without incident. When I got it back in the garage I switched it off (including the key fob). Waited a few minutes and tried to start it again. Same thing: click, whirr, lights dim, engine will not turn.

Either the battery is dead - if so why, and why did it manage three starts that day before suddenly expiring?

Or, it is draining somewhere due to a fault.

Or, the alternator isn't charging it in the first place.

Unfortunately the dealership isn't open on Mondays so I'll have to wait until Tuesday. Hopefully they'll be able to take a look at it then. It'll start no problem off jumpleads or a booster pack and get me to the dealership. Will report back in due course.

Sounds more like the terminal is loose
Check with volt meter as it clicks or tighten bat bolts or terminal loose on cable
Hth

Terminals were rock solid. I refitted them myself after the winter lay-up. I disconnected them today when I put the battery on charge to elliminate any parasitic drain. Within an hour the tender was indicating full charge. I suspect the battery is not reaching full voltage but I'll know more tomorrow.

I should have reported back on this. Mechanic at my local dealership (not the dealer I bought the bike from) put a multimeter on and said the alternator is producing full output but battery is probably goosed. It hasn't misbehaved since but he said the voltage was dropping below normal levels on cranking. I noticed when removing the battery that the screws at the base of the battery box were very rusted as though the battery had leaked and corroded them, though there is no sign of leaking or bulging on the battery itself.
The bike was delivered by the dealer I bought it from with loose and coroded terminals and the day after delivery I went to start it and had a no-show. I suspect that the original dealer never connected the battery securely or bothered to grease the terminals when they prepped the bike for sale to it's first owner and powdery sulphate corrosion was forming and dropping down into the battery tray.