SN, sodium nitrite, works by absorbing oxygen from wherever it can get it (e.g. your blood), which converts the SN to sodium nitrate. So SN will soon disappear from your body, certainly in much less than 3 - 4 weeks. Sodium nitrate will not disappear, and a higher than normal concentration of sodium nitrate would suggest that you have used SN. However, my guess - and it's only a guess - is that if we really are talking about a period as long as 3 - 4 weeks, a body will have decayed so much that it would be difficult for them to demonstrate conclusively that the deceased person had used SN. Incidentally, 25 grams is a very large dose. You can ctb reliably with much less - study the many posts on the subject on this site - and if you want to leave as little evidence as possible it would seem sensible to use only what is needed to do the job, plus a bit more to be sure. If you want a method that doesn't look like suicide, go for a walk in the wilderness in winter. Lots of people do that and die, even though they had no intention of dying.