As already mentioned in other comments, your requirements are contradictory. You cannot have a gas that is non-existent at normal conditions but is also toxic to humans and has a distinctive odour at the same conditions.
Depending on when do you want the gas to present (e.g. to be toxic to humans or to be recognizable by its odour) and when do you want it to be absent (e.g. to be harmless or to be undetectable), you might need to introduce a change (like a switch) in your ambient conditions.
If you want the compound to be toxic first and non-existent later, you may consider very volatile compounds (gases, or liquids with a high vapour pressure), which are rapidly dispersed, e.g. by the wind. Thus, you can obtain a toxic concentration of the gas first and also can achieve concentrations below the detection limit later. Several chemical warfare agents are classified as non-persistent agents (i.e. they are likely to be used to kill and incapacitate rather than to deny access to ground) and thus suit this purpose. A typical example for such non-persistent agents is phosgene. It also has a distinctive odour, which is described as resembling freshly-cut hay or grass.
You may also consider a compound that is sensitive to heat and/or sunlight. Thus you could obtain a toxic concentration under dark or cold conditions (e.g. at night) and decompose the gas later using heat and/or sunlight (e.g. after sunrise). However, I don’t know any typical example compound that fulfils these requirements, is also sufficiently volatile and toxic, and has a characteristic odour.
If you want the compound to be absent first and toxic later, you may consider taking advantage of the different vapour pressures of compounds in dependence of the temperature. A typical example for this is the use of sulfur mustard (mustard gas) in winter, e.g. contamination of snow. Sulfur mustard freezes at about 14 °C and therefore detection systems (including the human nose) relying on detection of vapour can fail. If contaminated snow is carried (e.g. sticking to the boots of persons) into buildings, vehicles, or other warm places, you can achieve a toxic concentration in the air. The odour of sulfur mustard is described as resembling garlic, mustard, or leeks; however, the smell is actually caused by impurities of the technical product.
If you want the compound to be absent first, you may also consider using a chemical reaction to generate the toxic compound only when you need it. A typical example is sarin. Modern binary sarin weapons do not contain any sarin; they actually contain methylphosphonic difluoride, isopropyl alcohol (propan-2-ol), isopropylamine (propan-2-amine), and another compound (which is fortunately missing in most cookbooks that are circulating in the internet, and I prefer to ignore the details here, too). The compounds are mixed when the weapon is used (e.g. upon impact of the warhead) and react to the actual sarin. Pure sarin is odorless; however, the smell of impure sarin is described as resembling mustard or burned rubber. Furthermore, the characteristic smell of the precursors would be present (e.g. the fishy, ammoniacal odour of isopropylamine).
