Scam? Home alarm system reps

I recently moved back to the east side of town (Northshore, Channelview area), and our home has external security cameras. They are very visible. In the past two weeks, three different people claiming to be ADT reps have come to the door, alleging free upgrades and system checks. Now, I don't let anyone I don't know in my home or on my property – period – so I had them go about their business. However, I asked for a business card or flyer, and every rep was "out." I said they could email it to me (using a spam address I have handy), and they declined. This all reeks of scam energy.

However, what's the objective? I offered my email, thinking they'd want my info, but they didn't. Are they just trying to gain access to my home? Has anyone else had this experience?