In the absence of antibiotics, there is selective pressure to eliminate the genes or mutations that confer resistance. However, there are opposing factors - antibiotic resistance genes are often carried on plasmids, independent genetic elements that can be exchanged between bacteria. Plasmids that contain antibiotic resistance genes frequently also contain heavy metal resistance genes[1]. This allows for co-selection, where the presence of, for example, mercury pollution from coal-fired power plants will maintain antibiotic resistance in antibiotic-free environments[2,3]. Plasmids can act as parasites on their bacterial hosts - many carry pairs of genes that encode both a toxin and a less stable antitoxin. If the plasmid is lost, the bacterium will die[4]. All of these mechanisms can maintain antibiotic resistance without antibiotic exposure.
[1]: http://www.cell.com/trends/microbiology/abstract/S0966-842X(...
[2]: https://link.springer.com/article/10.1007/s00284-012-0194-4
[3]: http://www.tandfonline.com/doi/abs/10.1080/01490450902889072
[4]: http://www.annualreviews.org/doi/abs/10.1146/annurev.micro.5...