The answer is simple. Anywhere your eyes can see, any where your eyes can hear, and every where your attention is.
To provide more substantial substance to this question, these are common places that you can find advertisement.
- Web Browser
- Social Media
- Mobile Device
- Your Mailbox
- In the sky (airplanes, drones, etc)
- Inside Locations such as bathrooms, dining, and more
- Shopping Carts
The list goes on. Just a rule of thumb, if there is a blank space, an advertisement piece may find its way there.