Origin of using "clocked" to mean "noticed"

The OED’s definition of the verb in this sense is ‘To watch or observe; to look at, notice. slang (orig. U.S.)’ and the earliest citation, from an American source, is dated 1942. At roughly the same time the verb is also first found as meaning ‘to punch in the face’. ‘Clock’ is first recorded as a noun meaning ‘the human face’ in 1923. 36 years later it is found as a noun meaning ‘a punch’.

We speak readily enough of the face of a clock, so I suppose the extension of ‘clock’ to mean a human face is not so very surprising.

Oddly enough, it originates from the USA (according to OED)

For origins, I have heard to two explanations. One is pretty much what Barrie said - clock is figuratively a face, so to 'clock' someone is to notice their face.

The second is based on the origins of 'clock', (OED ~ "Middle English clok(ke , clocke , was either < Middle Dutch clocke (modern Dutch klok ‘bell, clock’), or < Old Northern French cloke , cloque = Central French cloche ‘bell’"), and an alternative use for bell/clock that was to have it tied around the necks of cattle to make them easier to locate, or 'clock'.

Fitting either of those explanations into something that apparently originated in the USA in the 1940s is problematic...

Of the many clock definitions in A Dictionary of Slang and Unconventional English (2002) by Eric Partridge are:

clock, n. ... 2. A face: from ca. 1870, ex US. Cg. dial.

clock, v.t. To time by a stop-watch: from ca. 1880: sporting s. > ca. 1910, coll; now verging on S.E. - ... 3. To watch (someone) patiently; c.: since ca. 1930 (Norman.) Perhaps ex sense 1. - 4. To catch sight of, to notice: c., mostly prisons': since ca. 1935. Frank Norman, in Encounter, 1959. 5. To "clock" someone is to follow someone and see what he backs. This is sometimes expressed as "Get on his daily" [i.e. tail; rhyming s. on Daily Mail] (Sunday Telegraph, 7 May 1967, anon. article on bookies' s.): racing: since ca. 1930. Cf. sense 3, and clock and house.

Similarly, The Concise New Partridge Dictionary of Slang (2007) includes:

clock noun ... 6 the face UK, 1918...

clock verb 1 to catch sight of or notice someone or something; to watch someone or something US, 1929. 2 to watch someone patiently; especially to follow someone with the purpose of discovering the details of a bet UK, 1958. ... 8 to punch, to strike with the fist. Perhaps, originally, 'to hit in the clock (the face) UK, 1932...

I've always thought of "clocking" something as becoming aware of the thing, similar to "having it on one's radar". It seemed to me to be related to clock positions:

https://en.wikipedia.org/wiki/Clock_position

So to "clock" something would be to get the thing on your metaphorical clock, i.e. to know where it was in relation to yourself. It's easy to imagine military/prison slang adopting terminology based on this concept.

'Klok' in Scandinavian languages has various meanings to do with 'wisdom' and 'knowledge' (http://www.meaningspage.com/norwegian-meaning/translate-to-english/wise#define)

Scandinavian immigrants to the U.S. could easily have slipped the word into colloquial conversation from their native language, a bit like the way the Welsh slang word 'but' (meaning mate/friend) became the widespread 'buddy' through usage by Welsh immigrants, or 'thaler' from Russian became the word 'dollar'.

Often people can be more than a little English-centric when trying to analyse American English, really it's more of a melting pot.