Ya, some reasons they may be able to enter without a warrant are 911 calls and clear sign of distress. Now the latter can be interpreted different ways... and "911" calls can be placed by anyone...
I once lived under my dealer, in a small 5 apt complex with my girl at the time.
We were "busy" in the bedroom for a while, no where near any phones.
We come out, take the weed out and start burning some bong hits (she's a mom and was a school teacher at the time but son was at the father's place for the weekend)
All the while not knowing the cops are outside the door (they've been knocking but we couldn't hear from the room) - And they knock yet again.
She goes to the window and looks, panics thinking it's about her son - opens the door and stands in the doorway.
They just push her aside, three cops walk in - half Oz and bong on the coffee table, fresh smoke in the air, kids toys still everywhere... Did NOT look good...
"There was a 911 ca- Hey I smell weed!", Were the first words outta the first cops mouth...
AND evidently if they can see a bong or paraphernalia out in plain sight they can see through your window or the ajar door, then I think they can do something about that too. Not just that, but also the smell...