Yes, other teammates will be alerted to the use of a reboot van in two ways: (1) they will hear a loud charging sound during the reboot if they are near enough to the van - it even shows up on the visual sounds feature, if enabled, and (2) if a reboot is successful, a loud horn/siren sound plays and a large pillar of light shoots into the sky as your teammate glides back into play. Also, keep in mind that once a reboot van is succesfully used, there is a cooldown for a short period of time before the van may be used again.