If you cannot smell or see any signs of coolant under the bonnet esprcially at the rear and the two pipes going through the bulkhead grommet to the heater look ok (and grommet intact) then its possible to see the pipes from the inside as they enter and go into the matrix.they are only sealed with o rings which could leak .you may be able to see signs of leakage or smell it more down there. If its the actual matrix then due to the awkward job it is to do a replacement properly(without cutting dash) there has to be no harm in trying a leak sealer .after all its going to do one of two things. Either seal the leak but may cause a slight blockage that would reduce heat output a bit or it could do nothing .if the matrix is past its best you would not be damaging a good part that would have to be replaced anyway in the future.
Also its worth noting ,has this smell been there for ages or just started happening and is it accompanied by any misting up/steam. I had one in a viva that misted up the windscreen every time you turned on the heater but only for the first minute.you could smell it slightly but no water loss.when i removed the matrix it was only the slightest bit discoloured in one corner but enough to seep a very very small amount of steam.