A few months ago, what looks like mouse droppings began to appear on my front porch. I’ve tried putting out something called Zep Commercial Weatherproof Rat and Mouse Bars. They have no apparent effect. Can someone recommend something that works?
Well, if the droppings are outside, what’s the problem? 
As long as Mousie’s out in the Great Outdoors, why worry?
Mouse bait won’t work outside because Mousie has too many other options for Fun And Tasty Stuff To Eat. As long as he’s living outside, in a natural state, he’s going to stick to eating what he knows.
My recommendation: get a dog and have it hang out on the porch occasionally. Warned by the scent, Mousie will quit hanging out on the porch himself, and he’ll move inside the house, and then you can use your mouse bait. 
Orkin, call Orkin. If it was inside your house, or if you knew where they were coming from, it would be different. Actaully, your city may even deal with it for you. We had a deck in our back yard and the city told us that if we ever ripped it out to call them so they could check under it for mice etc. Well when we finally did rip it out, we found a rat nest under it and the city came and took care of it. (I should mention that this was at my parents house and I don’t know any of the circumstances surrounding the fact that the city asked them to do this. There may have been a rat problem in that area).
Anyways, so if I were you, I would check with Orkin, check with your city, and look into products that will drive the mice away not bring them closer.
Does that help???