As Keanu walks down an alleyway towards the entrance to a warehouse building (with connecting way bridge in front, above him) there is a bright red (indefinable) object at ground level on his left:
when next shot shown from now in front of him about to enter the building, the object is no longer seen.
Diamonds of this colour and size are exceedingly rare. One would be hard pressed to find a single round one on the market worldwide. If someone offers you 10 of them all round every expert in fancy coloured diamonds would immediately know that they are fake without needing spectral analysis.
The town in "Siberia" (actually shot in Marquette, Manitoba Canada) has green street signs that are obviously of North American design and would not be present in the real Siberia.
Keanu Reeves states: "I packed for St. Petersburg." Meaning that he packed for warm weather. St. Petersburg Russia is slightly farther north than Juneau Alaska so if it is winter in Siberia it would most certainly also be winter in St. Petersburg (the average January temperature in St. Pete. is 22F) so the St. Petersburg scenes were very obviously filmed in summer (the trees are fully leafed out).