In Star Wars, when R2-D2 and C-3PO leave in the escape pod, the star destroyer operator says no life forms were detected. Given the technology in the Star Wars universe, I'm surprised they couldn't see that there were two droids, especially since they were both still active. If they were deactivated I could understand. Are there other instances of droids evading scanners in a similar manner?
If a high ranking imperial officer had been present, this mistake would probably not have taken place. Imperial troops and operators have had a history of being unimaginative throughout the original trilogy*. This is probably because they are usually acting on information provided on a need-to-know basis. It is likely that the mission objectives given to them say nothing about the theft of the Deathstar plans (a fact best kept secret!).
- Letting the escape pod go.
- Falling for the 'need some help here' call from inside the Millennium Falcon.
- Ignoring small fighter assaults on the Deathstar (leaving Vader and his personal squad to engage them).
- Trusting two unknown droids during a security breach.
And so on and so forth.
Droids are hunks of metal with robotics and circutry. So are spaceships and escape pods. It wouldn't be very easy to pick out a droid from the vessel it's in unless it's in the middle of an empty cargo bay or something, so in the confines of an escape pod you might not be able to distinguish them.
Also note that in that particular situation, the troopers said no "lifeforms" were on board. That tells me they weren't looking for droids; remember they seemed a bit surprised when they found out that there were droid tracks on the surface.