FAQ: Royal First Class Lounge and Royal Orchid Spa information and access.
The 700 baht is the departure tax which is paid by every traveller on a ticket purchased travelling out of Thailand where they have been in Thailand for more than 24 hours. It is not charged on tickets which only transit as you don't enter Thailand. If you make the decision to leave the airport on transit you are then liable for this tax as it wouldn't have been collected on your ticket. This is what the signs are letting you know. Saying that it is rare that it is collected but not unheard of.