My S 200 runs it's under-seat outlet through the ignition so no draw while it's off, I'd assume your S125 is the same, but could be an odd ignition fault so easy to check & something to look at.
Beyond that anything that's aftermarket is suspect, but most likely an old battery.
Get a trickle charger & leave it on that when sitting see if that gets it. Those are worth having even if the battery is new if you don't the bike ride daily.
Even if it does "fix" it and the battery's old replace it at some point in the near future, or get real good at using your kick start.