Flash operations, exif data - anyone know the meanings of "return detected" or "not detected"

I have been exploring with flash control using the NEX7 pop up flash and bouncing it etc while also using a linked off camera synching on second flash. I like this for flexibility but am puzzled when I look later at the exif data recorder:

I tend to see Flash "On, Return detected"

and "On, Return not detected"

But can't see the pattern with my pic taking to work out what the difference is.

Does it relate to the metering on preflash OR the main flash, I wonder?

PLUG: By the way I use Rob Cole's excellent & free ExifMeta plugin for LR 3 (&4) to enable easy viewing of selected exifdata in the LR Library module. I'd rather look at in in develop module but Rob says that capability (ShowBiz) is LR4 only... He has many other great tools (and a very clever supporting elare software framework and is very responsive) so check him out, perhaps.

http://www.robcole.com/Rob/index.cfm

There are lots of possible states and I dug up a declaration of the tech stuff about the data values but so far no clue about what the results actually tell me, hence my post.

TAG_FLASH public static final int TAG_FLASH 0x0 = 0000000 = No Flash 0x1 = 0000001 = Fired 0x5 = 0000101 = Fired, Return not detected 0x7 = 0000111 = Fired, Return detected 0x9 = 0001001 = On 0xd = 0001101 = On, Return not detected 0xf = 0001111 = On, Return detected 0x10 = 0010000 = Off 0x18 = 0011000 = Auto, Did not fire 0x19 = 0011001 = Auto, Fired 0x1d = 0011101 = Auto, Fired, Return not detected 0x1f = 0011111 = Auto, Fired, Return detected 0x20 = 0100000 = No flash function 0x41 = 1000001 = Fired, Red-eye reduction 0x45 = 1000101 = Fired, Red-eye reduction, Return not detected 0x47 = 1000111 = Fired, Red-eye reduction, Return detected 0x49 = 1001001 = On, Red-eye reduction 0x4d = 1001101 = On, Red-eye reduction, Return not detected 0x4f = 1001111 = On, Red-eye reduction, Return detected 0x59 = 1011001 = Auto, Fired, Red-eye reduction 0x5d = 1011101 = Auto, Fired, Red-eye reduction, Return not detected 0x5f = 1011111 = Auto, Fired, Red-eye reduction, Return detected 6543210 (positions) This is a bitmask. 0 = flash fired 1 = return detected 2 = return able to be detected 3 = unknown 4 = auto used 5 = unknown 6 = red eye reduction used

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NEX 7, 5N, OSS SEL18200, kit1855, 16mm & 12mmUWA, Sigma30 plus various manual glass