Even when the currency sign is dropped, you would usually have a notation like 12.—, 12.--, or 12.00 (or with commas instead of periods as decimal separator depending on locale) to signify that it is a price and not just a random number. You could easily guess that from the context, but it is a deliberate circumvention of convention nonetheless. Using just '12' also signals that fractional units of that currency are not something that establishment deals with (which is fine for Japanese Yen or South Korean Won, but comes across as rude for Euro, and most Dollars and Pounds).