1. Security alarm will sound if (a/the) door is opened. (Can 'door' also be an uncountable noun here because you don't know if it is one door or two doors?
2. So 'door' can be uncountable and there is a possibility that 'a/the/this' is omitted?I agree. a or the or even this has been omitted, probably to save space on the sign
Thanks in advance.