Dan Gillespie, Jeff Hilndrand and Richard Higgins, co-founders of Hotdrop.

Spies and secret agents have used the “dead drop” for years. Roll up a little note, then stick it in the ground for another person to pick up. Essentially, hide the message in plain sight to share information with desired respondents — the idea is nothing new.

Combine that with a mobile hot spot, though, and it’s something unique: a Hotdrop.

Forget leaving a note for someone; the mobile application — set to premiere next year — will allow users to leave a piece of content, such as a photo or message, for later passersby who walk through the same area, said Jeff Hilnbrand, Hotdrop co-founder and junior mechanical engineering major.

“With the app, users can make connections through spaces,” Hilnbrand said. “Anything from messages for a pickup basketball game, scavenger hunts or event invites for students in a certain location — it’s a way of allowing people to communicate based on where they are in the world.”

Hilnbrand and two other university students, Richard Higgins and Daniel Gillespie, came up with the idea at the University of Pennsylvania’s PennApps hackathon in early September. Since then, they’ve founded the Hotdrop company and have been developing the mobile application.

The concept stemmed from the idea of a “local chat” — a way to communicate with people nearby in real time, whether at a riot, protest or other large gathering. The startup snowballed from there, transforming into the idea of “dead drops” and “hot spots” for social purposes. A user could leave a picture of a place from 10 years ago or construct something silly revolving around the space, Hilnbrand said.

The team has “Hotdrop time” three nights a week in the university’s Startup Shell, a student-run innovation space that offers technological and resources to entrepreneurial students. And though members set their meetings from 6 to 10 p.m., the team will often stay until the early morning, sometimes until 4 a.m. to iron out kinks and develop the project.

Hotdrop is one of about 30 active startups in the Shell, Hilnbrand said. Other active groups revolve around entertainment installations, a collaborative music app, an educational app for entrepreneurship and new hardware for 3-D printing technology.

The Hotdrop team developed an early version of its app at the University of Pennsylvania’s hackathon with a fourth team member, Evan Wang, and chose to continue to develop it following the competition after receiving positive feedback from corporate sponsors and developers.

“Location services and messaging have been around, but combining them in a usable way would really be an avenue to new experiences,” Hilnbrand said. “I won’t say we aren’t tinkering and playing around with other projects, though.”

The team members have already developed early alpha prototypes of the application, and they plan to make a “private alpha” version available to fellow students at Startup Shell in the next week. They expect to release a public beta version to the university community by 2014.

“That’s the interesting thing about app development — you can create something you think is awesome, and everyone might hate it,” Hilnbrand said. “We’re building and setting parameters for what we think is best, but we’re definitely excited to get more opinions from people outside of our team.”

In the spirit of the Shell, Hotdrop hopes to make its app collaborative and organic. The team plans to open an application programming interface, which would allow programmers to build on Hotdrop’s platform.

“Let’s say someone wants to use our code and make it so every time someone walks through a space, they immediately call their parents,” Hilnbrand said. “They could program off of it and make the app different and better than what we could think of.”

Working together to improve plans and bounce ideas off one another is nothing new to Hilnbrand and the rest of the Startup Shell members. The space is an incubator for ideas and an ideal collaboration space, said Brent Bovenzi,a member of the student team that focuses on areas of operation within the Shell.

“It’s a place where you want to go to work on something — maybe on a project, maybe on a company — and other people are, as well. That in itself, having feedback from others immediately, makes everything better,” said Bovenzi, a junior electrical engineering and government and politics major. “It’s different skills coming to the same space and contributing their own strengths to every project.”