Pro Star Wars geek.
Reason number 57,164,657 of why the English language sucks.@ImperfectPixie has the right idea. In the phrase "Guest Entrance", the word 'guest' is being used as an adjective (happens a lot in English, like Eiffel Tower or pedestrian crossing). Adjectives rarely are declined in the plural form (look at the yellows flowers!). Another way of constructing a similar sentiment is to create an adjectival form for 'guest', such as 'Guestal Entrance'.
In the phrase "Guest without bags", it is usually understood that since "guest" is being used as a noun, then the noun should be declined to take the plural form, "guests."
However, you, @G00fyDad, are interpreting the whole phrase as an adjectival phrasal modifier of "line". You're thinking, "this is a guest line", more specifically, "a guest without bags line." In that case, then, yes, "guest" used as an adjective and thus would be singular in form.
Of course, you're entirely wrong for throwing an understood "line" onto the phrase.