lets say you have 6 cards, all motion detector.
1. you play the first motion detector, but you cannot scan a room, since you havent played a tech card yet.
2. you play the second motion detector, you now can scan 1 room, since you've already played a tech card before this card.
3. even though you've played 2 tech cards already, when you play your third motion detector, you still only scan 1 additional room.
in total, you'll be scanning only 5 rooms. 1 for each card, minus the first card.