At the beginning of the film as Maxim and his companion are returning from the gardens they drive past a group of cyclists wearing modern helmets. The cycling helmets in the 1930s, while rare, were not the solid covering style.
When Maxim and Mademoiselle (the future Mrs. DeWinter) are discussing her leaving and never seeing him again in his hotel room, you can see the mic pack on Maxim (Armie Hammer) attached to his pants.
(at around 1h 35 mins) Jack Favell blackmails Maxim with a note allegedly containing an invitation from Rebecca to meet up on the night of her death. However, the note is undated, so there is nothing that links it to the events of that particular day.