Mysterious H

There's an enigma in this fair city (Brussels). The red letter H on a smallish, square sign, affixed to many buildings on the street-side. We noticed it right away and are not sure what it is for. They are all over the city. One clue is that there is always a metal lid or trap-door in the sidewalk, below the sign. Theories:

1) Definitely doesn't mean "Hospital"

2) Access to a natural gas line?

3) H for "Hydrant" in case of fires?

4) Secret trap-door for small people to get into the bomb-shelters buried 30 meters below the buildings?

Will let you know when we have found the answer...

--RP--