Hi
You don't specify the type of system you have as sealed systems these days
normally suggest 'Combi ' boilers but conventional systems can be sealed.
That said the task is still the same Find the leak!
A few pointers .Have you had any work done recently (floors removed/refixed?
Moved or relocated any large objects? Had a new carpet fitted?
Think back to when the problem first started (if known) and try and relate
anything done around that time.
Moving a wardrobe caused a leak once when the new location crushed a badly
fitted rad pipe in a bedroom.
Carpet fitters (sorry to all good carpet fitters) have IME sometimes
punctured a pipe whilst fitting edge trims or loosened rad conections
causing a minor leak which the new carpet absorbs.
The most common is creaky floor boards,you screw them down only to clip a CH
pipe.
All are a pain to locate.

HTH
CJ