When I was in that situation a little while ago the RSPCA inspector came out but said there was nothing he could do for THREE days and he couldn't break in without a police officer.
The poor dog was going beserk inside, but luckily there was an open window and a neighbour was able to feed and water the dog through the window so it wasn't an 'emergency'.
The RSPCA guy put sellotape over both the front and back door so he could see discreetly if they had been opened....luckily the owners came back before the three days were up.
If there are no open windows though it's a serious situation...contact the RSPCA asap to get the ball rolling, ask a few neighbours if they have seen the people in the last day or so, maybe scout round the house looking for any open windows etc. Try looking through the windows to see if you can see anything - if you talk to the other neighbours first and get them involved if possible you won't get nicked if anyone sees you scouting round the building!
Good for you for being concerned, I hope you can get it sorted, keep us posted!