I doubt that the correct term is a manhole - i.e. large enough for someone to physically enter a chamber below the cover at floor level. This is more likely to be a rodding point, i.e. for the purposes of clearing a blockage or simply for inspection/check/test purposes. Much depends on how the thing is covered or capped off - if it is a sealed cover then the normal state (closed) renders it as good as non-existent until opened. Under such circumstances it is no worse than, say, a shower drain with a trap on it or even less obtrusive if it has a sealed cover that is fixed in place. If the cover is fixed in place then nothing should be able to come through, neither liquid nor gases. If merely sitting there by gravity then potentially (in extreme circumstances) the cover could be lifted off by a surcharged (i.e. full to overflowing) pipeline.