Young and Innocent

Christine Clay (Pamela Carme), a successful actress, argues passionately with her jealous ex-husband Guy (George Curzon), who makes particular reference to Robert Tisdall, a young man staying near her at her retreat on the English coast. Christine slaps him several times across the face, but he hardly reacts, choosing instead to depart without a word.
The next morning, Robert Tisdall (Derrick De Marney) happens to be walking along the seaside cliffs when Christine's body washes ashore. He runs to get help and call the police, but two young swimmers arrive just in time to see him racing away from the corpse. A belt from his raincoat, which had just recently gone missing, is found next to the body, further implicating him in her strangling. He is subsequently arrested and becomes the main suspect, partly because of a large sum of money Christine left to him in her will, a gift he was unaware of.
Tisdall is grilled all night by Scotland Yard detectives until he faints. He is brought round by the aid of Erica Burgoyne (Nova Pilbeam), daughter of the local police Chief Constable and who was one of the women who witnessed him running away from Christine's dead body. Tisdall from there is taken directly into court for his formal arraignment. Saddled with a completely incompetent barrister, Tisdall doubts if his innocence will ever be established. He elects to take advantage of a crowded courthouse and make his escape, wearing the barrister's eyeglasses as a disguise.
After his escape, Tisdall finds Erica Burgoyne on the road pushing her car, which has run out of gas, and convinces her to give him a ride in her Morris car. Though she is initially fearful and unsure about her passenger, Burgoyne eventually becomes convinced of his innocence and elects to help him in any way that she can. They are eventually spotted together, forcing both to stay on the run from the police. Tisdall tries to prove his innocence by tracking down the stolen coat to show that it still has its belt.
The duo succeed in tracking down Old Will (Edward Rigby), a sociable china-mender and homeless bum that was known to have received Tisdall's coat. He agrees to help them find the man who gave him the coat; unfortunately, all that Old Will can remember about the man is his distinctive eye twitch.
Upon searching the pockets of the coat, Erica finds a clue, a box of matches from the Grand Hotel, a place Tisdall has never been to. She is separated from the group, however, and taken in by the police. Upon realising that his daughter has fully allied herself with the murder suspect, her father chooses to resign his position as Chief Constable rather than arrest her for assisting Tisdall. Nonetheless, Erica and Old Will go to the Grand Hotel together, hoping to find the true murderer. In a memorably long, continuous sequence, the camera pans right from Erica and Old Will's entrance to the hotel and then moves forward from the very back of the hotel ballroom, finally focusing in extreme close up on the drummer in a dance band performing in blackface. Recognizing Old Will in the audience, and seeing policemen nearby (unaware that they have followed Old Will in the hopes of finding Tisdall), the man performs poorly due to fear and a drug he has been taking to try to control the twitching, and is berated by the musical conductor. Eventually the drummer faints in the middle of a performance, drawing the attention of Erica and the policemen. Immediately after being revived and confronted, he confesses his crime and begins laughing hysterically.

Question: What is the clue Erica finds while searching the pockets of the coat?
Answer: Box of matches