The dungbomb is hidden among others on the sidetable next to the middle bed, in front of the pitcher.
On Ron's bed, which is in the middle, the curtains on the canopy completely collapse when you mouse over them.
If you mouse over the painting of the Chudley Cannons Quidditch team, you can just make out that they wave.
You can hear people shouting in the background.
At the beginning of the moment, you can see moving shadows in the doorway and hear footsteps, which stop before playing an excerpt from the chapter:
“Now, really, enough’s enough!” Professor McGonagall was back. She slammed the portrait behind her as she entered the common room and stared furiously around. “I am delighted that Gryffindor won the match, but this is getting ridiculous! Percy, I expected better of you!” “I certainly didn’t authorize this, Professor!” said Percy, puffing himself up indignantly. “I was just telling them all to get back to bed! My brother Ron here had a nightmare —” “IT WASN’T A NIGHTMARE!” Ron yelled. “PROFESSOR, I WOKE UP, AND SIRIUS BLACK WAS STANDING OVER ME, HOLDING A KNIFE!” Professor McGonagall stared at him. “Don’t be ridiculous, Weasley, how could he possibly have gotten through the portrait hole?” “Ask him!” said Ron, pointing a shaking finger at the back of Sir Cadogan’s picture. “Ask him if he saw —” Glaring suspiciously at Ron, Professor McGonagall pushed the portrait back open and went outside. The whole common room listened with bated breath. “Sir Cadogan, did you just let a man enter Gryffindor Tower?” “Certainly, good lady!” cried Sir Cadogan. There was a stunned silence, both inside and outside the common room. “You — you did?” said Professor McGonagall. “But — but the password!” “He had ’em!” said Sir Cadogan proudly. “Had the whole week’s, my lady! Read ’em off a little piece of paper!”