I think it's quite likely that 9079 IS your man...
The Medal Card of 9079 Pte Alexander Dunn, 2nd Bn A&SH, shows that he was discharged on 12/1/1916 according to King's Regulations Para.392 Subpara.xxi (i.e."on the termination of his period of engagement").
FindMyPast has the Militia attestation of one Alexander Dunn of Old Kilpatrick, born c.1884, son of John of Scots Land (in 1902, which I think matches the 1901 census, if I've got the right fellow?)
He enlisted in the 4th (Extra Reserve/Militia) Bn A&SH on 13/1/1902, but transferred "To Arg.&Suthd.Hrs." (i.e. to a Regular battalion) a year later on 13/1/03, and the number 9079 is consistent with a regular enlistment in Jan. 1903 (no.9076 William Stewart joined 2/A&SH from the Black Watch Militia on 9/1/03; and he too was discharged time expired on 8.1.16).
His period of engagement with the Regulars would be 12 years - normally 7 with the colours plus 5 in Reserve, so Alexander would have been transferred to the Reserve (sent home but liable for callup) on 12/1/10, and his 12 years would have expired on 12/1/15 if there hadn't been a war!
On the outbreak of war he would have been called up, and his total service period extended for one year, taking us to 12/1/16 (the date on the MIC), after which he would have to be discharged.
The card does seem to match your man!
Adrian