Strong intermitent sewage smell in bathroom, possible broken underfloor pipe
Although your question is not detailed or specific enough to provide definitive answers, you should do these things:


Check the void space that you mentioned for signs of a sewage leak
Have a plumber run a camera down your branch and main sewer lines to check the integrity
Check your vent lines to ensure they are not plugged or restricted
Check for a drain that is not frequently used (like a hidden floor drain) and run some water down it to establish a seal in the trap if necessary)


Consider this:


It is not uncommon for drain flow(s) from another appurtenance to cause a siphon action in a tub or sink drain, resulting in sewer gases/odors entering the room. This is caused by inadequate venting and there are methods to abate it, call a plumber.