You're so right in that the entire SMITH family evaporates sometimes after 1851
. This may be another case of alien abduction!! 
I'm a bit surprised that Robert LAURIE doesn't appear in 1861 either but then is married in 1863 saying he's been in Whitburn. 
k I've looked for some clues but nothing is jumping out at me. Any chance they all emigrated??? One thing was quite clear though....Agnes was shown as "married" on the 1851 while her hubbie was shown as married to someone else..... 
I assume this is Robert in 1881...
Dwelling: Newbigging
Census Place: Carnwath, Lanark, Scotland
Source: FHL Film 0203623 GRO Ref Volume 632 EnumDist 3 Page 8
Marr Age Sex Birthplace
Robert LAURIE M 40 M Carluke, Lanark, Scotland
Rel: Head
Occ: Tailor
Catherine LAURIE M 39 F Lesmahagow, Lanark, Scotland
Rel: Wife
Sarah LAURIE 12 F Newton Ayr, Ayr, Scotland
Rel: Daur
Occ: Scholar
John Rattray LAURIE 9 M Pennycuik, Edinburgh, Scotland
Rel: Son
Occ: Scholar
Margaret LAURIE 6 F Dalkeith, Edinburgh, Scotland
Rel: Daur
Occ: Scholar
Agnes LAURIE 4 F Dalkeith, Edinburgh, Scotland
Rel: Daur
Sorry...no other suggestions at the moment...
Best wishes
Jean