I didn't try chase all the examples down in the movie itself, but according to the script at http://academy.filminfocus.com/scripts/ttss_screenplay.pdf Smiley is established as being in the habit of placing a wedge at the top of his door when he leaves - as a method to know if someone other than him has opened it. This is later extended to encompass 'other spies' in a James Bond-style habit.
He's seen to check for it in two scenes, then a later time he finds it on the front step rather than in its correct place - he therefore concludes someone has been inside… or is still there.
Any other 'clever' spy would be expected to have found it & correctly replaced it.
36 EXT. SMILEY’S HOUSE - EVENING
Smiley on the steps to his home, stops, examines the door, removing a WEDGE from the top of the door frame.
37 INT. SMILEY’S HOUSE - EVENING
Smiley walks into the hallway, stopping to pick up post
38 INT. SMILEY’S HOUSE - EVENING
Smiley places the wedge and the letters on the mantlepiece,
where a BUNDLE of LETTERS already sits, addressed to ANN SMILEY. Unopened
50 INT. CONTROL’S FLAT - HALLWAY - DAY
Smiley and Guillam reach a flat near the shadowy top of the building. Smiley studies the door. He runs his fingers over the door-frame and there it is - a little WEDGE at the top.
87 EXT. SMILEY’S HOUSE - DAY
Smiley walks up to the steps of his home, stops, examining the door, his absent-minded air hardening into something more ALERT. He scans the step and stoops to pick something up. A WEDGE.
88 INT. SMILEY’S HOUSE - DAY
Smiley walks into the hallway senses alert. He pads towards the drawing room, wipes sweat from his hands on his trousers, steels himself. Is it finally Them, come for him? He walks in and finds…
89 INT. SMILEY’S HOUSE - DAY
RICKI TARR sits smoking in the darkness, the knife on his lap.
RICKI TARR Hello Mister Smiley. You remember me.
SMILEY Hello, Ricki.
He tosses the wedge onto the table.
SMILEY (CONT’D) You missed a wedge.
RICKI TARR Losing my touch.
Smiley watches him closely, very aware of the knife. Tarr rubs his beard.
RICKI TARR (CONT’D)
I know what I must look like. Rip Van Winkle. Feel like him. I could sleep for a year. Two years.
SMILEY Where’ve you been, Ricki?
RICKI TARR Here, there. Run out of places now. They’re gonna kill me.
SMILEY Who is?
RICKI TARR Your lot. Or their lot. Whoever gets me first. I’m innocent, by
the way. Within reason. (Beat) You going to find him then? The Mole?
Online scripts are known to not always be exactly the version that made the final cut of the movie, so this may not be verbatim.
I had a quick scan through the movie; found the first and third occurrence but not the second…
I've pushed these quite hard in Photoshop to make it easier to see