Proto-MSA

*sar - behind (Kogan 2015:569)

The origin of Proto-MSA *sar ‘behind’ is uncertain. Contra W. Leslau and M. Bittner (1914:15), any connection with PS *ˀaṯar- ‘trace’ can be safely excluded for phonological reasons. Quite far-fetched is the comparison between the MSA terms and Arb. sarīr- ‘the part where the head rests upon the neck’ (Lane 1339), sarāt- ‘back’ (ibid. 1353), Amh. säräsär ‘vertebra, spinal cord’ (AED 487) suggested in SED I No. 253. Shall one rather compare Hbr. swr ‘to turn aside’ (HALOT 748), assuming a semantic development from the more original meaning “to turn back”? (Kogan 2015:569)

Mehri
sār - behind; back, backwards (ML 351)
Likely related with a meaning shift.
Jibbali
sér - behind; back, backwards (JL 231)
Likely related with a meaning shift.
Soqotri
sɛr - behind (LS 290–291; CSOL I 644; CSOL II 577; Naumkin et al. 2015a:84; Naumkin–Kogan 2021 :527)

di-sɛr ‘three days after today’ (CSOL I 1:11)

di-sɛr sέri ‘four days after today’ (CSOL I 1:11)

be-di-sɛr ‘after’ (spatial):

sábaḥ wa-sábaḥk be-di-sέreʸh ‘He (the billy-goat) jumped, and I jumped after him’ (Naumkin et al. 2022:270)

ḷe-sɛr ‘behind’:

ḷe-sɛr ḷe-ṭeḳ wa-ḷe-ˁóubɛb wa-ḷe-ʸhérom ḷeḥaľítens ‘Behind the ṭeḳ and the ˁóubɛb and any other tree, I will provide for them’ (CSOL II 5:2829)

ḷe-sɛr ‘after (temporal)’: 

wa-ṭáhɛr ˁaf yaˀáraḥ ḷe-sɛr múγreb diˀáḷ ḳáˁar di-ˁážeʰ ‘He set off and that evening (lit. after sunset) reached the house of a woman’ (CSOL II 13:10)

men sɛr ‘behind’: 

hímaˁk bíľeʰ teḥarɛsímin men sɛr térbak wa-šɛ́kidk ‘I heard something rustle behind the pen and I was scared’ (CSOL I 10:1);

men sɛr ‘after’: 

men sɛr ˁóuṣar wa-ˁáṣar enáḳoḷ men šeˀefóti ‘It is only from time to time that I go to milk in šeˀefóti’ (Naumkin–Kogan 2021:527)