Apple may still be tight-lipped about the features coming in the iOS 9 mobile operating system, but there's already evidence that the tech company is publicly testing the software on the iPhone 6, iPhone 6 Plus, iPad Air, and more.

According to a report from mobile app marketing platform Fiksu, 145 distinct IDFAs (Identifier for Advertising) came from iOS 9 devices so far in 2015.

The IDFA is a way for mobile advertisers to uniquely identify devices based on the operating system and type of device. The research shows that 92% of these iOS 9 devices (or 134 devices) hail from the U.S. while 2 others were seen in China, 2 more in the Czech Republic, and 1 each in several other global countries. The variety of countries could indicate that Apple is experimenting with those markets and how each device operates in those international markets.

As for the devices themselves, the breakdown is clear. 63% are using the iPhone 6 or 6 Plus, 12% are on the iPad Air 2, 12% are on the iPhone 5s, and 13% makeup the remainder devices including the iPhone 4s, 5 and 5c, and the iPad 4, Air, mini 2, and mini 3.

These findings coincide with Macworld's data from last week after the Apple-centric news site received a tip from a reader about versions of the iOS 9 software showing up in his site's web analytics. The Macworld editors checked out their own stats and found 10 pageviews from devices running some version of iOS 9 over the last few months.

AppleInsider did its own research and found a similar story with a growing number of hits from iOS 9 devices over the last month.

In fact, Fiksu and Macworld both say that the earliest showings of iOS 9 in their web analytics goes back to September 20 of last year, only three days after iOS 8 was released. While some point out that this could be a glitch in the way some analytics software interprets iOS 8, it could also mean that Apple isn't wasting time testing out the next version.

Fiksu's research went on to explain a bit more insight into these iOS 9 devices, reporting that 71% of app events came from social networking apps, while 16% were games, 5% were Lifestyle apps, 3% were fitness apps, and the other 5% were from various uncategorized groups.

Another report at Forbes describes changes coming with iOS 9 as indicated by leaked information on the upcoming iPhone 6s smartphone. According to the leaked info, the iPhone 6S will have more RAM to accommodate the needs ofthe new iOS 9 software.

As Forbes points out, "One year tends to be about changing how iOS works, the next about adding features and services on top of it. With iOS 9 the leap should again be about how iOS works, especially as it merges ever closer with OS X. Currently iPhones don't need to be faster with how efficiently iOS 8 operates, but moving to RAM twice as fast and doubling the amount of it in the iPhone 6S and iPhone 6S Plus suggests major change is afoot."

Since Apple traditionally announces new software and products at its WWDC conference in June every year, we can expect to see the iOS 9 news ramping up considerably between now and then.