It is always possible to recover deleted data from ANY storage medium - even RAM, although that wears down quickly relative to disk storage.
When binary is stored as 1's and 0's, they never quite make it to exactly 1 or 0. More like 0.95, 1.05 or 0.05, etc. By examining the specific device and what is exactly stored, you can determine what is "underneath". This holds true for several deletes.
But, that kind of forensic examination is way beyond the means of probably anyone here, and for us, basically only remains a nice thought...
That being said - secure data deletion requires 39 complex operations to make the data 100% unrecoverable (for hard disks). The only sure way to "delete" data designated "secret" by the DoD is to incinerate the drive.
So, at the end of the day, rest assured that your data can be recovered, but that you'll never be able to afford it