Bring small sealable bags to put your treasures in. That way your pockets will not get dirty.
Being older, I am always watching where I place my feet to avoid falls. Since not everyone cleans up after their pets when walking them, you don't want to step in that!
Also, not everyone leashes their dogs when walking them. These dogs are generally very friendly...BUT sometimes jump on you in their exuberance and can knock you down-been there, not hurt, but also not fun. Since the majority of people walk their dogs morning and evening, before and after work, you may wish to avoid those times.
Be mindful of your surroundings. You should be able to hear traffic as well as everything going on around you so don't listen to music, etc., on your walks. It is very easy to get excited and distracted when looking for and finding treasures!
Don't walk the same route every day. Make sure the alley is wide and wide open so you can clearly see your surroundings as well as be seen.
You may want to determine if the alley is city property or not. Some people may not want you on or near their property.
Always leave a note or let someone know where you are going.