Unless the legislation has changed it is not forbidden to use an airband scanner and listen into transmissions personally. The important part is *personally*, as you may not share any information gathered with anybody else.
1. airband scanner with speaker on that loud that another person may listen to it, too, is forbidden.
2. airband scanner used with earphone should be no problem, especially if you are sensitive enough to tuck away the scanner itself it your coat's pocket.
Actual enforcement may vary with location and even with personnel within a specific organization, like local police in one location might not bother but airport security in another location might do.
Having said that, I saw no security guys at TXL
's visitors terrace on Saturday, but you have to pass a security check anyhow to enter the terrace. But the guy operating it on Saturday let us pass with all normal electronic items, so I suppose an airband scanner would pass, too.
BTW, entrance fee of 2 € at TXL
's visitors terrace is forfeit at the moment as long as the vending machines are broken.
Carpe diem ! Life is too short to waste your time ! Keep in mind, that today is the first day of the rest of your life !